Defense Information Systems Agency
"[DISA] performs a number of very important missions in support of the President, the Secretary of Defense, the Joint Chiefs of Staff, the Combatant Commanders, and the other Department of Defense (DoD) components under all conditions of peace and war. Some of these missions are designated as core missions because together they provide highly integrated C4 warfighting capabilities. The whole is greater than the sum of the parts and removing one of the core missions would adversely affect the others. Some other critical missions are designated as "best fit" missions, meaning that DISA is well-suited to perform these missions and has been assigned them over time. However, they could be assigned to others without destroying the synergy that exists among the core missions. Thus, the terms "core" and "best fit" are not designators of relative importance but indicate degrees of synergy. The designated core missions of DISA are communications, joint command and control, defensive information operations, combat support computing, and joint interoperability support."

Source:
http://www.disa.mil/main/coremission.html